The low temperature γ-irradiation of norbornadiene and (1,5-ciclooctadiene) platinum(II) dichloride in methyltetrahydrofurane matrix leads to a selective production of electron capture centres which were identified by ESR as platinum(I) diolefin radical anions. The Molecular Orbital calculations suggest that the unpaired electron is in a b 2 orbital (assuming an idealised C 2 r symmetry) where the 5d y z is the major metal orbital involved. The low contribution of 5d y z to the SOMO (α 2 = 0.23 as derived from the analysis of the experimental ESR parameters) implies a high delocalization on the ligands. The electronic structure of the paramagnetic species and, in particular, the large negative contributions to the calculated g-tensor elements giving rise from the low energy empty levels, were rationalised by extended Huckel MO calculations.